This invention relates generally to radio frequency (RF) amplification. More particularly, the invention relates to a system for reducing third order intermodulation distortion in broadband CATV RF amplifiers.
Lowering distortion in RF power amplifier circuits without compromising their transient response is an omnipresent problem. High frequency amplification is widely used in communications and broadcasting and also where high-speed switching is required for use in digital instrumentation. However, high frequency amplifier applications extend linear operation into areas where parasitic effects of interelectrode capacitance, wire inductance, stored charge and even operating frequency wavelength begin to adversely affect and dominate circuit behavior.
Minimizing distortion is particularly important when a series of amplifiers is cascaded over a signal transmission path, such as a series of RF amplifiers in a CATV transmission network. Disposed throughout a CATV transmission system are RF amplifiers that periodically amplify the transmitted signals to counteract cable attenuation and attenuation caused by passive CATV components, such as signal splitters and equalizers. The RF amplifiers are also employed to maintain the desired carrier-to-noise ratio. Due to the number of RF amplifiers employed in a given CATV transmission system, each RF amplifier must provide minimum degradation to the transmitted signal.
In an ideal communication system it is preferable that the components which comprise the system are linear. However, as a practical reality, there are many nonlinearities that are typically introduced by the electronic components, such as RF amplifiers. The distortions created by an RF amplifier which are of primary concern are second (even) and third (odd) order harmonic distortions. Prior art amplifier designs have attempted to ameliorate the effects of even order distortions by employing push-pull amplifier topologies, since the maximum even order cancellation occurs when the proper 180xc2x0 phase relationship is maintained over the entire bandwidth. This is achieved through equal gain in both push-pull halves by matching the operating characteristics of the active devices.
However, odd-order distortion is difficult to remedy. Odd-order distortion characteristics of an amplifier are manifest as cross modulation (X-mod) and composite triple beat (CTB) distortions on the signal being amplified. These are two types of intermodulation (IM) distortion. X-mod occurs when the modulated contents of one channel being transmitted interferes with and becomes part of an adjacent or non-adjacent channel. CTB results from the combination of three frequencies of carriers occurring in the proximity of each carrier since the carriers are typically equally spaced across the frequency bandwidth. Of the two noted distortions, CTB becomes more problematic when increasing the number of channels on a given CATV system. While X-mod distortion also increases in proportion to the number of channels, the possibility of CTB is more dramatic due to the increased number of available combinations from among the total number of transmitted channels. As the number of channels transmitted by a communication system increases, or as the channels reside closer together, the odd-order distortion becomes a limiting factor of amplifier performance.
The nonlinear properties of an RF amplifier can be described by a curve which can be expanded into a Taylor series as follows:
Uout=a1xc2x7Uin+a2xc2x7Uin2+a3xc2x7Uin3+a4xc2x7Uin4+a5xc2x7Uin5 . . . anxc2x7Uinn,xe2x80x83xe2x80x83Equation 1
where Uin is the input potential and Uout is the output potential and an is a factor that determines the magnitude of the term. It should be noted that a1xc2x7Uin is the 1st order term; a2xc2x7Uin2 is the 2nd order term; a3xc2x7Uin3 is the 3rd order term . . . and anxc2x7Uinn is the nth order term. The magnitudes of the individual terms are strongly dependent on the input signal and, therefore, on the level control of the amplifier.
If we have the following:
Uin=Axc2x7cos xcfx89itxe2x80x83xe2x80x83Equation 2
There exists for each term multiple combination possibilities of the input circulating frequencies xcfx89i due to the power of the corresponding order number. Additionally, in multifrequency transmission systems, such as a CATV transmission network, the number of new circulating frequencies at the output of the network increases exponentially with the number (i) of frequencies at the input. These new circulating frequencies are referred to herein as intermodulation (IM) products and are detrimental to the accurate acquisition of CATV signals.
Using the Taylor series, it can be demonstrated that all odd order terms create products which appear at the same location as the lower-valued odd order terms. Therefore, the third order term creates a product at the base frequency, (odd order number 1), the fifth order term creates a product at the third order and one at the base frequency. If the input signal consists, for example, of the base circulating frequencies xcfx891 and xcfx892 (i=2) with the same amplitude A, that is expressed with:                                                                         U                in                            =                              xe2x80x83                            ⁢                                                A                  ⁢                                      xe2x80x83                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  ω                        1                                            ⁢                      t                                        )                                                  +                                  A                  ⁢                                      xe2x80x83                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  ω                        2                                            ⁢                      t                                        )                                                                                                                                          =                                  xe2x80x83                                ⁢                                  0.5                  ⁢                                      A                    ⁡                                          (                                                                        ⅇ                                                      j                            ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              1                                                        ⁢                            t                                                                          +                                                  ⅇ                                                                                    -                              j                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              1                                                        ⁢                            t                                                                          +                                                  ⅇ                                                      j                            ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              2                                                        ⁢                            t                                                                          +                                                  ⅇ                                                                                    -                              j                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              2                                                        ⁢                            t                                                                                              )                                                                                  ,                                                          Equation        ⁢                  xe2x80x83                ⁢        3            
then the third order term a3xc2x7Uin3 of Equation 1, (which is of interest in the present application), creates the following new products: xc2x1xcfx891; xc2x1xcfx892; xc2x13xcfx891; xc2x13xcfx892; xc2x1(2xcfx892xc2x1xcfx891); xc2x1(2xcfx891xc2x1xcfx892). In this case there are 16 new circulating output frequencies due to two input circulating frequencies.
A xe2x80x9cweaklyxe2x80x9d nonlinear transmission system can be defined such that: a) the effect of odd order terms on other lower-valued odd order terms is negligibly small; and b) higher-valued terms after the third order term are negligibly small. Accordingly, a weakly nonlinear system may be mathematically described such that the Taylor series is broken off after the third order term (a3xc2x7Uin3). Weakly nonlinear systems are characterized in that a 1 db increase in the level of the input circulating frequencies xcfx89i causes an increase of 3 db in the third order IM products.
Communication systems, such as CATV systems which include broadband RF amplifiers, are further regarded as dynamically nonlinear systems whereby the amplitudes and phases of the IM products are dependent upon the input frequencies.
There are three basic ways of correcting distortion created by a non-linear device: 1) reduce the signal power level; 2) use a feed forward technique; and 3) use a predistortion or postdistortion technique. The first method reduces the signal power level such that the non-linear device is operating in its linear region. In the case of an RF amplifier this results in very high power consumption for low RF output power. Of course, the high power consumption is a disadvantage. However, this method is not an option if high output level is required on a permanent basis.
The second method is the feed forward technique. Using this technique, the input signal of the main amplification circuit is sampled and compared to the output signal to determine the difference between the signals. This difference is the distortion component which is amplified by an auxiliary amplification circuit and combined with the output of the main amplification circuit such that the two distortion components cancel each other. However, the power consumed by the auxiliary amplification circuit is comparable to that consumed by the main amplification circuit and the circuitry is also complex and expensive. At the upper frequency limit it is very difficult to maintain the magnitude and phase conditions with respect to temperature.
The third method is the pre- or post-distortion technique. Depending upon whether the compensating distortion signal is generated before the non-linear device or after, the respective term predistortion or postdistortion is used. In this technique, a distortion signal equal in amplitude but opposite in phase to the distortion component generated by the amplifier circuit is estimated and generated. This is used to cancel the distortion at the input (for predistortion) or output (for postdistortion) of the amplifier, thereby improving the operating characteristics of the amplifier.
The present invention is a distortion control circuit for selective modulation of an RF signal. The present invention includes an input port for coupling with an RF signal source, such as a multifrequency CATV signal, and an output port for coupling to an associated electrical circuit such as a hybrid RF amplifier. Hybrid RF amplifiers are known for generating unwanted nonlinear characteristics, particularly second and third order products. The present invention generates new third order products from the multifrequency RF signal which are the same magnitude, but opposite in phase to the nonlinear products generated by the hybrid RF amplifier. Since both the original multifrequency RF input signal and the new generated products from the invention are coupled to the input of the hybrid RF amplifier, the nonlinear products from the present invention and the hybrid RF amplifier will be canceled and the output of the hybrid RF amplifier will comprise only the multifrequency RF signal. The distortion control circuit includes a nonlinear circuit having a pair of diodes which are selectively biased to create a distortion product for adding to the input signal which is opposite in phase and equal in magnitude to any distortion created by the hybrid RF amplifier. The present inventive circuit is particularly adaptable weakly nonlinear systems and provides the ability to largely match the dynamically nonlinear behavior of a system to be compensated and achieve compensation over a frequency range of at least 860 MHz.
Accordingly, it is an object of the present invention to provide a circuit for reducing third order intermodulation distortion for a broadband RF amplifier.
Other objects and advantages of the system and the method will become apparent to those skilled in the art after reading a detailed description of the preferred embodiment.